With the arrival of very large scale integration (VLSI) in the semiconductor industry comes the need for lithographic techniques capable of producing clean, accurate exposures on resist-covered silicon wafers with minimum line widths of less than 1 micron.
At the present time, a wide variety of lithographic techniques are available to VLSI designers. They include contact, proximity, 1:1 optical projection, step-and-repeat reduction projection, X-ray and electron-beam systems.
Contact and proximity printing are the oldest types of lithographic systems available. In contact printing, a glass mask bearing an emulsion or chromium-film pattern is first aligned with reference points (alignment targets) on a resist-coated semiconductor wafer. Then the mask is pressed directly onto the wafer and exposed to ultraviolet light. In proximity printing, a gap of several microns divides the mask from the wafer.
The resolution of contact printing is limited only by the wave length of ultraviolet light used, so that 1 micron lines can easily be printed using this technique. Proximity printing has a slightly lower resolution which varies with the gap size. However, the yield of chips made with either contact or proximity printing is extremely low due to the mask damage and wafer contamination caused by contact between the mask and the wafer. Because of the poor yield, these techniques are gradually being phased out.
Optical projection techniques may be divided into two types. In 1:1 reflective optical systems, a complex reflective lens system uses a mask to project an image the same size as the mask onto the wafer. Because it does not touch the wafer and therefore cannot damage it, the mask can be made of a hard material such as chrome. Thus, 1:1 optical projection systems can achieve extremely high yields thereby eliminating the major drawback of contact and proximity printing. The primary problem associated with 1:1 projection, however, is the fact that if a wafer is distorted in processing, it will cause layer-to-layer registration of successive masks to be out of specification, reducing yield. Another drawback of 1:1 projection is the fact that line widths of only about 3 microns are obtainable.
In an effort to eliminate the shortcomings of 1:1 projection systems, step-and-repeat reduction projection systems utilize a smaller projection area. A UV light source is shown through a blown-up portion of a wafer pattern, commonly known as a reticle. The reticle's pattern is projected down through a reduction lens onto the surface of a resist-covered wafer. After exposure, the lens is mechanically stepped to a new sight for another exposure. This procedure is repeated until the reticle image is projected across the entire wafer surface. While step-and-repeat systems can achieve 1.5 micron line widths, they are much more costly than 1:1 projection systems. They also have an inherently lower through-put since they require many exposures rather than just one.
X-ray lithography is a form of contact printing in which an X-ray source replaces the UV source. Resists specifically designed for X-ray exposure are utilized. X-ray lithography can produce line widths of 0.5 to 2 microns. The masks, however, which must be opaque to X-rays, are made of gold deposited on a layer of silicon, Mylar or polyimide only 2-10 microns thick. Thus, they are very fragile. Other problems associated with X-ray techniques are distortion caused by ripples in processed wafers and lack of standardization in mask design.
Until recently, scanning electron-beam lithography has been utilized primarily in making masks and reticles for use in the above-described systems. However, as VLSI design rules move into the sub-micron range, direct writing on the wafer with an electron beam is becoming more prominent. According to this technique, a computer-controlled electron beam scans a pattern with extremely high resolution and accuracy across a resist-covered semiconductor wafer.
Three major advantages of such direct write scanning electron-beam systems are its 0.2-to-1 micron resolution, its ability to align a pattern to within 0.05 microns and its ability to correct for wafer distortion.
In each of the above-described lithographic systems except electron-beam direct write, an optical mask must be registered with the wafer. That is, the mask image must be accurately aligned with the wafer so that the pattern created by the exposure is properly positioned on the wafer. To accomplish this, a reference target formed on the mask is matched with an alignment target formed on the wafer. This matching is done either manually or automatically using through-the-lens techniques.
In an electron-beam writing system, the mask pattern is represented by data which is embodied in the software of the system. When the system is utilized to pattern masks, no alignment targets are required since the pattern is formed on a plate which is initially bare. The resulting mask may then be properly aligned with an underlying wafer. However, when the system is utilized for writing a pattern directly onto a wafer, it first must be registered with the die pattern which exists on the wafer.
To accomplish this alignment, the electron-beam system executes a low energy scan to locate an alignment target which has been formed on the wafer. The system is programmed to locate the alignment target by recognizing a specific predetermined wave-form pattern which is characteristic of the target. A number of targets may be utilized for rotational or other alignment adjustments. After the electron-beam system has recognized the target and is registered with the wafer, the system executes a high energy scan according to its software instruction set to pattern the wafer.
The edges of the alignment target must present a clear, high resolution image to the electron-beam system so that the system may be registered with the extreme accuracy required for producing sub-micron line widths. Furthermore, as stated above, the electron-beam system must be able to recognize a particular alignment target which is defined in the system software.
One type of alignment target utilized in electron-beam direct write systems is a recess of a particular geometry, typically a square or a rectangle, which is cut into the surface of the wafer and has steeply-sloped sidewalls. Steeply-sloped sidewalls are preferable to vertical sidewalls because sloped sidewalls provide some width to the edge of the target as the wafer is scanned from directly above by the electron-beam system. However, utilization of sloped sidewalls results in loss of resolution.